1. Field of the Invention
The present invention relates to a drive circuit for an ultrasonic motor for driving a rotor by a travelling vibration wave generated by an electro-mechanical energy transducer element such as an electrostrictive element, magneto-strictive element or piezoelectric element, and more particularly to such an ultrasonic motor drive circuit having a digital control circuit.
2. Related Background Art
Various drive circuits for driving the ultrasonic motor have been proposed in view of nature that the motor is efficiently rotated only when a signal at a resonance frequency of the motor is applied.
(1) An oscillator having several oscillation frequencies is used, the respective frequencies are applied to the ultrasonic wave motor (SSM), rotation speeds are detected, and the frequency which resulted in the highest rotation speed is selected and fixed.
Alternatively, the frequency is continuously swept instead of applying several frequencies and the sweeping is stopped when the rotation speed of the SSM reaches a highest speed and the frequency is fixed thereat.
(2) A detection terminal for detecting a drive status of the SSM is provided in the SSM and a signal at the detection terminal is fed back. A high Q filter around a known resonance frequency of the SSM is inserted in a loop of a feedback circuit. A loop gain at the resonance frequency is raised so that the resonance frequency of the SSM oscillates by the feedback action and the SSM is driven by the oscillated signal.
Alternatively, instead of raising the loop gain around the resonance frequency, the SSM is forcibely driven at a frequency around the resonance frequency and a signal produced at the detection terminal by the drive is fed back so that the frequency around the resonance frequency is exactly matched to the resonance frequency, and the SSM is driven by the signal of that frequency.
Those prior art apparatus have the following disadvantages.
In the apparatus of the type (1) which selects or sweeps the drive frequency, a circuit for selecting or sweeping the frequency is necessary and means for detecting the rotation speed of the SSM is also necessary, and the circuit configuration is complex. Since the resonance frequency of the SSM changes with a load applied to the SSM or an environmental condition, it is necessary to repeatedly sweep or select the drive frequency to continuously update the drive frequency in order to attain efficient rotation.
In the apparatus of the type (2) which uses the signal at the detection terminal of the SSM, a frequency which follows the change of the load applied to the SSM or the environmental condition is obtained, but the high Q filter or an oscillation circuit which forcibely drives the SSSM only at the start is necessary. Thus, tee circuit configuration is complex and a power consumption of the circuit increases.